"We are creating a world that all may enter without privilege or prejudice accorded by race, economic power, military force, or station of birth. We are creating a world where anyone, anywhere may express his or her beliefs, no matter how singular, without fear of being coerced into silence or conformity."

The freedom offered by anonymity was one of the great early promises of the internet. In cyberspace, the fat kid with glasses who was bullied at school wouldn't be overweight or shortsighted, because those categories wouldn't exist. As Facebook is now by some measures "more popular than Google", that promise would seem to have faded almost completely away. The pioneers of cyberspace who imagined that online and real-world identities could be kept separate reckoned without Mark Zuckerberg.

On Facebook, the opportunities the internet provides for performing a variety of shifting, contradictory identities are severely curtailed. Your identity on Facebook is circumscribed by the same conventions that define who you are in your offline social networks: your name, where you live, your gender, age, sexuality, marital status, political views, religion, who your friends are and what you look like – so much for Barlow's vision of not coercing people into conformity.

Most arguments about Facebook and privacy focus on how well the company protects this information, keeps it secret from other users, from advertisers, from identity thieves, from search engines, from the world at large. But there is a more fundamental question, which is why anyone should give so much personal information to Facebook in the first place. The company's motives in asking for it are plain enough: market researchers used to have spend a lot of time and money gathering this kind of information, and now here we all are, splashing it around for free while Zuckerberg and Co make a killing selling it to advertisers. But why are we so happy to give it up? And what's in it for us?

Zuckerberg's answer to the second question would be that the more Facebook knows about you, the more it can tailor your "experience" of the web to suit you. On the Facebook blog last April, he wrote:

"If you're logged into Facebook and go to Pandora [an internet radio station] for the first time, now it can immediately start playing songs from bands you've liked across the web. And as you're playing music, it can show you friends who also like the same songs as you, and then you can click to see other music they like."

It's a nice enough idea, in its limited way, though it misses one of the great points of radio, which is to expose you to music that you and your friends don't know already: there wouldn't be a place for someone like John Peel in Zuckerberg's universe.

Of course, the model that Zuckerberg is hoping to replace isn't the Peel show but the search engine. If Zuckerberg gets his way, Facebook recommendations will replace Google searches as the main route by which we navigate to websites. This would hardly be a paradigm shift; more a tidying up, combining what are currently two steps into one. With a search engine, you have to know more or less what you're looking for before you begin: there's an implied recommendation preceding most things we type into Google. What Zuckerberg's betting on is that those recommendations will increasingly be made online, with a direct link, so the work you now ask Google to do will already have been done.

As for the first question, why people are so unfussed by Facebook's lack of privacy, it must in part be because the site fosters the illusion that you're among friends. The success of Facebook implies that most people are more comfortable thinking of the internet as an extension of their offline lives, where everything and everyone they are likely to encounter is already known to them. Which is all very well, but it does mean that the overweight kids in glasses get bullied there too.

Sign up for the Guardian Today

Our editors' picks for the day's top news and commentary delivered to your inbox each morning.